Summary of Significant Accounting Policies - Foreign Currency Translation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Expense, Net [Member]
Foreign Currency Translation [Line Items]
Net Foreign Currency Transaction Gains (Losses)	$ (0.8)	$ 0.8	$ (0.2)